I used a Trezor Safe 5 to create a 20-word seed phrase and then wanted to import it into my Keystone device, which accepts SSS 20-word multiseeds. It failed to recognize the seed, though I am 100% sure I entered the seed correctly, I used Trezor Suite seed check, referencing the seed as it was entered into the Keystone, not from the writing on paper.
The issue is with the Keystone device, and my guess is that this is because it is a single seed, not a multi-seed.
There could be a compatibility issue if the Keystone device does not support single-seed implementations of SLIP 39, but multiseed only.
This would expose a HUGE problem because it means that, just because a wallet accepts SLIP 39 seeds, doesn’t mean it accepts both single and multiseeds, and this will cause an escalating crisis among users as more wallets adopt SLIP 39 while being nescient of this compatibility issue.
The compatibility problem you’re experiencing is not technically a distinction between single- and multi-seed; but there is an important bit set in there that makes it not compatible with old implementations.
New implementations, however, will already know about this bit.
So there is very little risk of “more wallets adopting SLIP39” and choosing to support only the old version without this new bit.
Based on everything I’ve learned so far I feel like you’re 100% right. If SLIP39 were to continue being updated, it would definitely cause waves of compatibility issues but it says on the Git page that they won’t change it and if they do it would be given a new title. So this particular issue I’m having should be fixed and put in the past. I sent the Keystone team an email and they showed concern so I’ve explained it to them based on what I’ve learned from you. Thanks!